The present invention generally relates to a semiconductor memory devices, and more particularly to a semiconductor memory device used as a digital buffer performing a delay of digital data and to a reading and writing method for the semiconductor memory device.
In an image data processing apparatus, such as a facsimile machine and a digital copy machine, which handles digital image data, a line buffer memory is used for storing the image data. The purpose of using the line buffer memory is to perform an image processing such as enhancing of an edge of an image. Such an enhancement is performed by comparing image data corresponding to a particular line with image data corresponding to other lines including one or two lines prior to the particular line or one or two lines after the particular line.
As long as the line buffer memory is used for the above-mentioned application, the line buffer memory may have only a function to read from and write to the same address. However, a conventional line buffer memory such as show in FIG. 1 has a function to read from and write to different addresses. Accordingly, the conventional line buffer memory comprising a semiconductor memory device is provided with two ports, one for reading and one for writing.
FIG. 2 shows a circuit diagram of a memory cell having a read port and a write port, the memory cell being provided in the conventional line buffer memory shown in FIG. 1. In FIG. 2, transistors enclosed by circles are provided for constructing a two-port type memory cell of a semiconductor memory device in addition to transistors necessary for constructing a single-port type memory cell. Due to these additional transistors, the conventional line buffer memory has an area size one and a half to three times larger than that of the semiconductor memory-device having a single port. Therefore, the conventional line buffer memory has a higher manufacturing cost than the semiconductor memory device having a single port.